1. Field of the Invention
This invention relates to the fabrication of hard disk drives (HDD), particularly to a method of controlling slider fly height by use of heater elements to increase head protrusion at the air bearing surface (ABS).
2. Description of the Related Art
As magnetic read/write heads have been required to deal with magnetic media having increasingly higher area density of their recorded information, various methods have been developed to improve the capabilities of the head to read and write at those levels. Traditionally, the direction taken in trying to achieve the reading and writing of this high density information has been to decrease the spacing (i.e. the static fly height) between the disk and the slider in each new generation of products. FIG. 1 is a schematic illustration showing a suspension-mounted slider (collectively termed a “head gimbals assembly (HGA)”) positioned above a rotating magnetic hard disk during disk-drive operation at ambient operating temperature. The suspension (101) holds the slider (10) at an angle above the surface of the spindle-mounted (300) magnetic disk (400), producing a magnetic spacing between the air bearing surface (ABS) (100) of the slider and the disk. The rotation of the disk is, by definition, into the leading edge of the slider, while the read/write head (600) is located at the trailing edge of the slider. The write portion of the head (90) is above (more to the trailing edge) the read portion (30). The hydrodynamics of the air layer between the ABS and the disk surface supports the slider at a static fly height above the disk.
However, the limit of the total clearance budget prohibits a continuous reduction of this static fly height. In addition to the static fly height variations from the ABS and HGA/HSA (head gimbal assembly/head stack assembly) manufacturing processes, other factors also contribute to the total clearance between the head and the disk. A simple example is the drop in static fly height when the HDD is moved from sea level to a higher altitude. Yet another example is the isothermal PTP (pole tip protrusion) associated with the change in ambient temperature of the environment in which the HDD is located. In this regard, Berger et al. (U.S. Pat. No. 6,707,646) discloses the deposition of a magnetoeleastic layer on the HDD suspension arm to increase or decrease fly height when the ambient temperature is increased. Furthermore, the writer coil induced PTP (protrusion caused by joule heating of the coil) also diminishes the fly height clearance when the coil is activated to produce magnetic flux in a HDD write operation. There is a clear necessity to have a method of producing DFH (dynamic fly height) control, i.e. a method of providing a controllable head-disk spacing under various operational conditions, to avoid incidental contacts between the head and the disk that result from these inevitable variations in static fly height.
A common prior art approach to introducing such a “dynamic” control of fly height spacing is to embed a thin layer of heater film inside the magnetic recording head. The heater film is electrically connected to the pre-amplifier within which a heater current is activated to increase the heater film temperature and, thereby, to increase the temperature of the surrounding materials of the head structure. When subjected to this increased temperature, the materials forming the head begin to expand in accordance with their respective thermal expansion characteristics. This leads to a thermally deformable ABS that achieves a lower spacing between the disk surface and the RG (read gap) and WG (write gap), thus greatly improving head performance.
When the read/write operation is not required, the heater current is turned off so that the ABS is elastically returned to its original, non-deformed state. The induced rise in temperature produced by the heating is sufficiently mild that the reliability of the head is not detrimentally affected. In addition, the heater activation has not shown a degrading effect on the magnetic reader in terms of noise and stability since the magnetic fields produced by the heater activation currents is minimal.
The prior art discloses several approaches to DFH that utilize heater elements formed within the head. Kurita et al. (U.S. Pat. No. 7,095,587) teaches a thin film heating element formed within the alumina overcoat layer. Oyama et al. (U.S. Pat. No. 7,086,931) shows a heater formed on each thin film magnetic head wherein the heaters are connected in series and a resistor is connected between each heater. Kamijima (U.S. Pat. No. 7,068,468) describes an upper and lower heat source and flexible layers located opposite the head ABS with respect to the write head. Koide et al. (U.S. Pat. No. 7,102,856) addresses the problem of conductively connecting the heater to external circuitry and discloses a pair of heater electrode pads positioned outside the recording and reproducing pads.
The utilization of the DFH heater shows an unequivocal improvement in HDD performance. However, the same DFH power setting cannot be expected to deliver the same changes in spacing for each individual head due to the inevitable variations in the manufacturing process. Obtaining precise tuning of spacing on an individual head-to-head basis requires an off-line calibration procedure to map out each head's DFH operation range from touchdown to zero power. The brief touchdown between the head and media has to be designed to carefully prevent any hazardous HDI (head disk interference) that may cause severe performance degradation and possible reliability failure of the HDD.
In addition to the touchdown related HDI hazard during calibration with the existing DFH design, the potential for HDI during HDD operation when the heater is activated is also troublesome. The goal of existing DFH design is to enable maximum actuation efficiency for the RG and WG. This actuation efficiency is defined by RG or WG protrusion per unit of heater power in nanometers per milliwatts (nm/mW). The placement of the DFH in a vertical direction, i.e. the direction from leading edge to trailing edge in the mounting of the head in the slider, within the structure of the head is also critical in achieving minimal point (MIN) clearance of the RG/WG to the disk. The MIN being defined at the point on the ABS in front of the heater at the overcoat.
A prior art DFH location within the overcoat alumina (aluminum oxide) was proven to have unacceptable RG-to-MIN clearance. In this prior art structure, the actuation efficiency at the minimal clearance point is too high compared to the RG actuation efficiency. The touchdown onto the disk is, therefore, premature because the RG spacing to the disk has hardly changed, resulting in unusable DFH during HDD operation.
Referring to FIG. 2, there is shown a schematic cross-section of a read/write head indicating an approximate placement position for a heater element. This placement is used to demonstrate the differences between the effects of a single source prior art heating element and the symmetrically positioned double source heater element of the present invention. In FIG. 2, proceeding in the vertical direction (which is not the direction of head fabrication), from leading to trailing edge of the head, there can be seen a slider substrate (10), a lower read shield (20), a read head (30), typically a giant magnetoresistive (GMR) or tunneling magnetoresistive (TMR) read head that includes a read gap (RG) that is not specifically shown, the head being formed over the lower read shield, an upper read shield (40) formed in a horizontal plane over the read head, a layer of insulation (50) separating the two read shields, a further layer of insulation (60) formed over the upper read shield, a DFH heater element (70) formed over the upper read shield and embedded within the layer of insulation (60), a double layer of coils (80), shown in square cross-section, that serve to inductively activate the magnetic write head and a magnetic pole structure (90) typical of a perpendicular magnetic recording (PMR) type write head (for exemplary purposes only). The pole structure includes a write gap (WG) layer (95). The ABS plane is indicated as (100), and a slider substrate is indicated as (10). An overcoat layer (200) typically covers the trailing edge of the head. It is understood that the ABS (100) is coated with a protective hard diamond-like coating (DLC), but this is not shown. The existing placement of the DFH heater (70) above the upper read shield (40) works well in delivering good RG (30) actuation (“actuation” denoting the effects of heat-produced variations in height relative to an undistorted ABS that occur when the heating element is activated) while still causing greater protrusion of WG (which is closer to the trailing edge) than RG. The shape of these heat-produced height variations along a given linear direction, called an actuation profile (shown in FIG. 6a and FIG. 6b and discussed below), accomplishes the result that the greater protrusion of the WG effectively provides a recessed region for the RG, so that the RG is further away from a HDI hazard during DFH operation. However, the higher actuation of the WG and WS (write shield) now causes them to become the minimal clearance point that will make the initial contact with the disk when an HDI occurs, either during off-line touchdown calibration or incidental bumping during actual HDD operation. A simulation of the DFH actuation profile in the cross-track direction shows a sharp protrusion of the WG and WS right at the track center (see FIG. 4 for these protrusions in a single element prior art heater). The excessive wear caused by DFH overdrive (excessive reduction of head-to-disk clearance) during touchdown calibration as evidenced by (for example) algorithms simulating acoustic emissions and write faults, would cause the DLC (diamond-like carbon) overcoat to be worn off, exposing the underlying magnetic material to the environment without any protection. The lingering reliability concern that magnetic material corrosion can lead to HDD write failure calls for an improved DFH design to resolve the issue. It is the purpose of the present invention to provide such a design.